In the field of lean-burn internal combustion engines, reduction of NOx, such as NO and NO2, in exhaust gas is a well known problem. It is known to use so called Lean NOx Traps (LNTs) in the exhaust system for adsorbing NOx when the engine is run lean and to convert NOx into N2 (Nitrogen gas) during a regeneration period when the engine is run rich. Here lean refers to an air-fuel ratio above lambda equal to 1, i.e. the stoichometric air-fuel ratio, where the production of HC (hydrocarbons) and CO (carbon oxides) are low and where the production of NOx is high. Rich refers to a value below the lambda equal to 1 and where the production of HC and CO is high and where the production of NOx is relatively low. However, NOx does not have to be low at this running mode, but the oxygen O2 level always is. The hydrocarbons and carbon dioxides are reductants in the regeneration process. Reductants may also be called reducing agents or reducing compounds. An additional example of a reductant is hydrogen gas H2.
When the engine is run rich the excess amount of reductants reacts with the adsorbed NOx and forms N2. The engine is then run lean again and NOx is again adsorbed until the engine is run rich.
The optimum NOx conversion temperature value in the LNT is dependent, for example, on such factors as which fuel is used, how the LNT is constructed and the materials used, the age of the LNT, etc. However, the optimum NOx conversion temperature value is chosen in a temperature region where optimum conversion of NOx into N2 is possible. The temperature of the LNT is dependent on the temperature of the exhaust gas which becomes higher when the number of revolutions in the engine increases and/or when the load on the engine becomes higher and/or when the engine is run rich.
One problem with prior art occurs when the engine is run at conditions where the temperature of the LNT is well below the optimum NOx conversion temperature. At this low temperature the conversion of NOx is low, but it is known one scenario where to start regenerate at the low temperature by running the engine rich and continue running rich until the temperature has increased to the temperature close to or on the optimum temperature for regeneration. One problem with the latter solution is that the NOx conversion is low per se at this temperature and the process is thus a waste of fuel. Another problem is that NOx adsorbs well at low temperatures and desorbs with increased temperatures, why the procedure would yield an amount of desorbed NOx that cannot be converted and thus will escape to the environment with the exhaust gas.
Another known scenario is to wait for the engine to produce exhaust gas with temperatures high enough to heat up the LNT to the optimum NOx conversion temperature. However, as described above, the NOx adsorbs well at low temperatures and desorbs with increased temperatures, why this procedure would yield an amount of desorbed NOx that cannot be converted at the optimum NOx conversion temperature and thus will escape to the environment with the exhaust gas.
Therefore, there is a need for an improved method and apparatus for NOx conversion when the engine is run at conditions where the LNT temperature is well below the optimum NOx conversion temperature.